This invention relates generally to the field of optical attenuators and more specifically to a fast optical attenuator.
Optical attenuators are used to adjust the optical power level in optical signals, for example to equalize levels among WDM channels or to prevent saturation of receivers. Such applications generally require the attenuator to be adjustable over more than 20 dB. Low cost examples of this type of optical attenuator include neutral density filters, attenuating prisms, beam blockers, tilting mirrors and/or systems to bend or off-set optical fibers. Typically, the relatively large mass of the movable attenuating optics in these devices limit the response time to milliseconds.
Optical attenuators may also be used to impose information upon the optical signal passing through them, for example telemetry information. However, such applications typically require a response time of microseconds with a dynamic range of about 5 dB. Conventionally, optical attenuators that are capable of achieving fast speed in the order of MHz are based on electro-optics, acousto-optics, or magneto-optics. Traditionally, these fast attenuators are wavelength sensitive, have a relatively high insertion loss, and a relatively high cost.
Ideally, an optical attenuator should have a fast response time and be able to adjust the optical power level over a wide range of intensity levels. Unfortunately, the design of a low cost attenuator that has simultaneously high speed and wide dynamic range is very difficult.
It is an object of this invention to provide to provide a fast optical attenuator that is relatively low cost.
It is another object of this invention to provide an optical attenuator having a wide dynamic range and that can also provide attenuation that varies at high speeds.
The instant invention provides a variable optical attenuator that attenuates over a wide dynamic range and that operates at high speeds. The optical attenuator includes a slow attenuator having a large dynamic range and a fast attenuator having a smaller dynamic range. In the preferred embodiment, the slow and fast attenuator are not coupled via optical waveguides, but rather are optically coupled via free space or are integrated in the same device. Advantageously, this compact arrangement reduces the excess loss associated with optical fiber coupling and lowers packaging costs. Moreover, each of the slow and fast attenuators work together to modulate an optical signal from the other such that the total attenuation of the device is the product of the two separate attenuations.
In a preferred embodiment, the fast attenuation is provided by a varifocal mirror. Electrostatic deflection of the mirror defocuses the optical system and attenuates an optical signal launched to the mirror from an input port to an output port. Since the mirror is generally limited to deflections less than 1 micron, fast attenuation within a small ( less than 5 dB) dynamic range is achieved.
In accordance with the invention there is provided an optical attenuator comprising an input port and an output port, a deformable membrane having a reflective surface disposed to reflect light launched from the input port substantially towards the output port, the membrane supported by a frame having an opening, and deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines the optical coupling efficiency between the input and output ports, the membrane deformable between a first position in which light launched from the input port is received at the output port with substantially no attenuation, and a plurality of other positions in which light launched from the input port is inefficiently coupled into the output port.
In accordance with the invention there is provided an optical attenuator comprising a first port and a second port, a deformable membrane having a reflective surface disposed to reflect light launched from the first port substantially towards the second port, the membrane supported by a frame having a radially symmetric opening, and deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines the optical coupling efficiency between the first and second ports.
In accordance with the invention there is provided an optical attenuator comprising: an input port and an output port; a deformable membrane having a reflective surface disposed to reflect light launched from the input port substantially towards the output port, the membrane supported by a frame having an opening; deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines a first order optical coupling efficiency between the first and second ports; and a movable member for controllably moving the frame and membrane between a plurality of positions that determine a second order optical coupling efficiency between the first and second ports.
In accordance with the invention there is provided a variable optical attenuator comprising: a first attenuator having a dynamic range less than about 10 dB and a response time of less than about one microsecond; and a second attenuator having a dynamic range greater than about 10 dB at a response time greater than about one millisecond, wherein the first and second attenuators are optically coupled in series.